1. Field of the Invention
The present invention relates to a termination structure of such a cryogenic cable as a superconducting cable, and particularly to a structure for coupling an end portion of the cryogenic cable to an end portion of a current lead connected to an external electric-current system which is at a room temperature.
2. Description of the Background Art
A normal-conducting cable has its end portion where the normal-conducting cable is raised perpendicularly to the longitudinal direction of the normal-conducting cable to be linearly connected to an end portion of a current lead extending to an external electric-current system.
As for a cryogenic cable, the cryogenic cable thermally contracts when cooled with liquid nitrogen for example while it thermally expands in a case where the cable has to be increased in temperature due to some accident for example. Under these situations, the end portion of the cryogenic cable is displaced in the longitudinal direction of the cable. The end portion is thus provided with a blocking part that blocks the movement of the cable in the longitudinal direction. Accordingly, the end portion of the cryogenic cable is linearly connected to a normal conductor and then to an end portion of a current lead provided perpendicularly thereto so as to form an L-shaped joint.
The end portion of the current lead is also displaced in the longitudinal direction of the current lead due to thermal contraction when cooled with liquid nitrogen for example, thermal expansion when heat is generated due to electric current flowing therethrough, or the like. In such a case, the joint portion could be broken, leading to impairment of the reliability of the whole system.
Japanese Patent Laying-Open No. 8-265956 for example discloses a method of absorbing thermal contraction and thermal expansion of a cryogenic cable. According to this method, the end portion of the cryogenic cable is made movable and the end portion is slid in accordance with an amount of thermal contraction or an amount of thermal expansion of the cryogenic cable so as to absorb the amount of thermal contraction or expansion.
Specifically, the end portion of this cryogenic cable includes a mechanism for sliding the end portion in the longitudinal direction of the cryogenic cable and means for measuring any change of the cryogenic cable that occurs due to thermal contraction thereof for example, and the end portion is slid according to the measurement. More specifically, the sliding mechanism for sliding the end portion in the longitudinal direction of the cryogenic cable has wheels on a joint portion between the cryogenic cable and a current lead and this joint portion on the wheels accordingly moves on rails.
The above-described method, however, requires the mechanism for sliding the end portion of the cryogenic cable in the longitudinal direction of the cable, i.e., wheels and rails for example, as well as the means for measuring a change of the cable caused by thermal contraction for example of the cable. Therefore, the structure is complex and an additional operation is necessary for sliding the end portion in the longitudinal direction of the cable according to a measurement, resulting in such problems as deterioration in reliability and increase in cost for manufacture and maintenance. There arises a further problem that, since the current lead is also slid in the longitudinal direction of the cable, a connector portion between the current lead and an external electric-current system encounters a compression force for example due to the sliding in the longitudinal direction of the cable.
In addition, the above-described method aims to absorb thermal contraction and thermal expansion in the longitudinal direction of the cryogenic cable, and does not aim to absorb any displacement in the longitudinal direction of the current lead. Thus, this method does not solve the problem of the breakage of the joint or connector portion due to any displacement in the longitudinal direction of the current lead and the resultant impairment of the reliability of the whole system.